Documenting ‘undocumented’ features
MMayerl asks over on the MSDN wiki, why we don’t document the undocumented C# keywords __makeref, __reftype, __refvalue and __arglist?
Because that defeats of the purpose of them being undocumented.
They are undocumented for a reason; so that the C# team is free to change and remove these keywords without worrying whether customers have taken dependencies on them. Documenting them conveys an implicit support agreement between Microsoft and the user, thereby preventing the team from ever changing the keywords.
We want less people to use them, not more.